As mobile device technology continues to develop and demand therefore continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which have high energy density and voltage and exhibit long lifespan and low self-discharge rate are commercially available and widely used.
In particular, lithium secondary batteries used in electric vehicles etc. require high energy density, high short-term power output, and use for 10 years or longer under harsh conditions in which charge and discharge are rapidly repeated under high current and thus need to have excellent output characteristics and longer lifespan than existing small lithium secondary batteries.
As a negative electrode active material for lithium ion secondary batteries used in conventional small devices, a graphite based material among carbon based compounds that reversibly receive or supply lithium ions while generally maintaining structural and electrical properties, and that have characteristics such as a chemical potential almost similar to metallic lithium upon intercalation and deintercalation of lithium ions is mainly used.
However, a theoretical maximum capacity of a negative electrode including a negative electrode active material composed of such a graphite based material is 372 mAh/g (844 mAh/cc), and thus, capacity increase is limited. Accordingly, it is difficult to perform sufficient function as an energy source of fast changing next-generation mobile phones. Furthermore, since high rate discharge characteristics of a graphite based material are not superior, it is limited to apply the graphite based material to a power source that should rapidly supply high electricity, such as electric vehicles, hybrid electric vehicles, electrically-drive tools, etc.
In addition, since lithium metal considered as a negative electrode active material has very high energy density, high capacity may be realized. However, there are problems such as a stability problem due to dendrite growth upon repeated charge and discharge, and short cycle lifespan. In addition, use of carbon nanotubes as a negative electrode active material was tried, but problems such as low productivity, high costs, low initial efficiency of 50% or less were pointed out.
It was known that, as another negative electrode active material, silicon, tin or alloy thereof may reversibly occlude and release a large amount of lithium through reaction that forms a compound with lithium, and much research thereinto is underway.
Since, silicon, for example, has a theoretical maximum capacity of approximately 4020 mAh/g (9800 mAh/cc, specific gravity: 2.23) that is much larger than graphite based materials, silicone is a superior candidate as a high-capacity negative electrode material. However, the negative electrode material has disadvantages such as very large volume change during charge and discharge, and non-superior high-rate discharge characteristics.
In addition, when a battery is composed of an active material prepared by mixing the different material types, charging current is considered into one material during charge and discharge of the battery due to voltage difference between the different materials, which deteriorates lifespan characteristics of the battery.
Accordingly, there is an urgent need for a negative electrode active material that exhibits predetermined capacity, high discharge characteristics and lifespan characteristics.
In this regard, first, a negative electrode material formed by coating a crystalline carbon based compound with an amorphous carbon layer may be considered. However, in this case, energy density is improved, but it is difficult to anticipate high output due to a low ratio of an amorphous carbon based compound included in a negative electrode material. In addition, it is impossible to obtain lifespan characteristics of a desired level due to poor electrical conductivity of a coating layer.
In another embodiment, a negative electrode material coated with a crystalline coating layer by coating non-graphitizable particle surfaces with a graphitizable material among amorphous carbon based compounds and by firing the coated particles up to graphitizable temperature may be considered. That is, by crystallizing graphitizable materials of particle surfaces through high-temperature treatment, a negative electrode material exhibiting high electrical conductivity may be considered. However, in order to obtain a coating layer having electrical conductivity of a sufficient level, firing should be performed to considerably high temperature (2800° C. or more). In this process, considerable portions of non-graphitizable particles, as a core, may also be crystallized.
Therefore, there is an urgent need for technology to resolve such problems.